1. Field of the Invention
The present invention relates to a method for converting a full color image into a monochrome image, the full color image comprising pixels that have a color value and the monochrome image comprising pixels that have a grayscale value within a full range from dark to light. The invention further relates to a print system, configured to perform said method.
2. Description of the Related Art
Color is used in images for a variety of reasons. A first reason is to obtain naturalness in an image, such as in photographic images. A further reason to use color is to make the image esthetically pleasing, such as in architectural and computer graphics. Color is also used to enhance the contrast in images, such as in business graphics and maps, or to group parts that belong to a similar function in an engineering drawing. Furthermore color is use to draw attention to a certain part in a large image, such as by highlighting an important area or an area where changes have been made in a drawing.
Still, many devices can only render one color in various tones against a background of another color. These devices render monochrome images. Hardcopies of this kind are printed by developing marking material such as black toner or ink on a contrasting background, such as white paper. These hardcopies or printouts are not only less expensive than their colored counterparts, but are also often more robust. An additional advantage is that their perception is less ambiguous for the considerable minority of persons that suffer from some form of color vision deficiency, also known as color blindness. Obviously, full color rendering devices, that use more than one colored marker, such as full color print engines, may also be used to make monochrome images.
However, many images are electronically or physically available with colors. In their digital form, which may be obtained by scanning the image in a conversion device, this means that a pixel in the image is characterized by a value that has more than one component, such as by three components in an RGB-image or by four components in a CMYK-image. Other color systems, such as Lab and YCrCb, may also be used to specify the color value of the pixels. Conversion of values within one color system to values within another color system are well-known. When these images are presented for reproduction as a monochrome image to a rendering device, a conversion is necessary from the full color characterisation of the pixels to a grayscale characterisation. A grayscale comprises values within a range from dark to light, corresponding to a maximum to a minimum of marking material for each pixel in a hardcopy of the image or, equivalently, corresponding to an intensity characterisation of the pixels in the image from high intensity to low intensity. A problem one is faced with is how to map the large multitude of colors to the limited number of tonal values in the grayscale.
A very often used approach is to use a transformation that retains a luminance channel of the color information or a derivative thereof. Of course, the distinction between two different colors of similar luminance is lost in this transformation, which is particularly objectionable if these two colors are spatially adjacent. Effort has been directed to solve this problem by making a cluster analysis of colors occurring in the image and optimise an agreement between a distance of colors and corresponding distance of grayscale values, as described in EP 1 292 112 B1. Another approach is given in US 2008/0181491 A1, wherein a high pass filtering of a chrominance component of the color image is used to modify the monochrome image that is based on a luminance component. Both approaches aim at a natural representation of the image in the form of a monochrome image, wherein color contrast is preserved.
In the function of highlighting parts of a large image by the use of color, the former approaches do not work. Although colors are mutually discernable, the luminance of the colors are on the grayscale between the dark value that corresponds to black colors and the light value that corresponds to white colors. In fact, all colors turn to gray and do not stand out amongst non-colored parts of the original image. Therefore, the highlighting function of the colors is lost in rendering a monochrome representation. Hence, a problem exists in transforming a color image in which color is used to highlight parts relative to non-colored parts, into a monochrome image.
An object of the present invention is to have a method for converting a full color image wherein colored parts attract attention relative to non-colored parts, into a monochrome image wherein the corresponding parts are highlighted.